(1) Field of the Invention
The present invention relates to a read technology for an optical disc drive, and more particularly, to a highly read light resistive optical disc drive.
(2) Description of the Related Art
Although the scope of the present invention is not limited to Blu-ray Discs (BDs), the following description assumes that the present invention is applied to BDs. Further, technical terms used with BDs are basically used.
Many of BD devices and other optical disc devices use a high-frequency modulation technology in order to suppress noise that is generated by a laser diode, which is used as their light source. This technology is disclosed in “Optics,” Vol. 14, No. 5, pp. 377-383, and well known to persons skilled in the art. Therefore, the following description furnishes only essential information about this technology without giving further details.
When laser light reflected from a disc is incident on an oscillating laser diode, the oscillation state becomes unstable so that significant laser noise is generated. The high-frequency modulation technology is used to avoid the generation of such laser noise. The high-frequency modulation technology is exercised so that a high-frequency signal is superimposed on a laser diode drive signal to generate a pulsed laser beam. The resulting light emission waveform looks like a periodic pulse shown in FIG. 2. Here, the ratio (duty) between a laser pulse interval (modulation period) and a light emission period and the peak power of pulses or the average power of a laser pulse train are used as main parameters. The laser pulse interval and pulse duty are determined on the assumption that a laser pulse reflected from a disc does not fall upon the laser diode during laser oscillation.
In a recording layer of an optical disc, information is recorded with marks and spaces. The marks and spaces used for a recordable optical disc differ in reflectance. Therefore, when pulsed laser light is focused on a recording film of the optical disc, the amplitude of a laser pulse is modulated because the intensity of reflected laser light varies depending on whether the laser light is incident on a mark or a space. If, for instance, no bandwidth limitation is imposed by a read photodiode and a current-to-voltage conversion amplifier, the resulting read signal waveform is as shown in FIG. 3. A signal made of a read pulse train indicated in FIG. 3 is hereinafter referred to as a pulsed read signal. The broken line in FIG. 3 indicates a read signal waveform that is obtained when, for instance, a laser continuously oscillates at the same output level as for a peak laser pulse generated during high-frequency modulation. In other words, the upper envelope of the pulsed read signal is shaped like a read signal waveform derived from continuous light. Therefore, a desired read waveform can be obtained by envelope detection that is, allowing the pulsed read signal to pass through a low-pass filter having a cut-off frequency sufficiently lower than the frequency of, the frequency of a high-frequency current to be superimposed. In existing optical disc devices, the above functionality is implemented as bandwidth limitation is imposed by an analog equalizer and a system having a photodetector and current-to-voltage conversion amplifier.
Pulsing a read signal is one type of amplitude modulation. Thus, a line-like spectrum of a superimposed high-frequency signal and a modulated read signal component, which is positioned near the line-like spectrum, are visible. In this specification, therefore, the superimposed high-frequency signal is hereinafter simply referred to as the carrier. A typical standard carrier frequency for BDs is approximately 400 MHz. The carrier frequency is determined exclusively by the optical path length of a read optical system. It is therefore understood that the carrier frequency does not significantly vary from one BD device to another.
FIG. 4 shows a typical spectrum of a pulsed read signal. The broken line in FIG. 4 schematically shows the bandwidth limitation imposed by an analog equalizer and a system having a photodetector and current-to-voltage conversion amplifier. When the above-mentioned conventional method is used to convert a pulsed read signal to a continuous signal, it means that the entire harmonic component is attenuated (suppressed). Therefore, the obtained read signal amplitude is reduced so that the ratio between the resulting amplitude and the amplitude of the pulsed read signal is substantially equal to a pulse duty.
A multi-tone demodulation (MTD) technology is available as a technology that provides improvement when the SNR (signal-to-noise ratio) is reduced by a decrease in the resulting amplitude as mentioned above. This technology is not only disclosed in detail in Japanese Patent Application Laid-Open Publication No. 2007-73147 (corresponding to US-A No. 2007/0053262), but also is described in “Novel HF-pulse read signal converter for increasing read signal SNR,” by Atsushi Kikukawa and Hiroyuki Minemura, Digest of International Symposium on Optical Memory 2007, pp. 302-303. The MTD technology not only improves the SNR of a read signal, but also addresses a problem in which the separation between a read signal and a carrier is difficult during a high-speed read as detailed in Japanese Patent Application Laid-Open Publication No. 2007-73147 (corresponding to US-A No. 2007/0053262). In other words, signals derived from MTD do not, in principle, contain a carrier's line-like spectrum.
It is well known to persons skilled in the art that an optical disc read signal process is mainly performed by a digital method such as a PRML (partial response most-likely) method. When a digital signal processing system is used, it is common that a PLL (phase-locked loop) circuit for synchronizing a read signal clock with a signal processing circuit clock is also digitized. In reality, however, the digitized PLL circuit includes analog components such as a voltage-controlled oscillator and a DAC (digital-to-analog converter). A problem with the use of an analog component is that its characteristics readily vary from one unit to another. In view of such circumstances, a signal processing system having no analog components has been studied in recent years as described in “Interpolation in Digital Modems—Part I: Fundamentals,” by Floyd M. Gardner, IEEE Transactions on Communications, Vol. 41, pp. 501-507 (1993).
Now that BDs are commercialized, an attempt is being made to further increase the capacity of an optical disc. As described in a paper authored by K. Mishima, D. Yoshitoku, H. Itoh, S. Yamatsu, H. Inoue, T. Komaki, K. Tanaka, and T. Aoi and presented at Optical Data Storage Top. Meet., 2006, TuA3, one promising method is to use an optical disc having multiple recording layers. When an optical disc having multiple recording layers is used, the amount of light reflected from the rearmost recording layer is extremely smaller than when a conventional optical disc is used because the light is reflected and absorbed by recording layers in front of it. If the ratio of the amount of reflected light to the amount of incident light is defined as the apparent reflectance, the apparent reflectance of a multilayered optical disc is lower than that of a double-layered BD or the like. This will lower the SNR (signal-to-noise ratio) of a read signal. Further, as the transmittance of each front recording layer needs to be sufficiently high, the absorptance needs to be low. It is therefore necessary that recordable discs be made of a recording layer material having a higher recording sensitivity than before. However, particularly in the case of a rewritable disc, an increase in the recording sensitivity will increase the likelihood of causing read light to erase recorded marks. In this specification, the expression “read light resistive” is used to indicate that read light is not likely to erase recorded marks.